FIG. 1 shows a schematic diagram 100 of elements from a BICM (Bit Interleaved Coding and Modulation) module within a receiver in a digital communications system such as a Digital Terrestrial Television (DTT) system. The demapper 102 receives cells 104 and uses noise variance estimates 106 in order to output soft information 108 (which may also be referred to as soft estimates), such as Log-Likelihood Ratios (LLRs). This soft information 108 is passed to the decoder 110. In some examples, soft information is fed back to the demapper from the decoder (as indicated by dotted arrow 112) and this is referred to as iterative demapping or iterative decoding. In such an implementation, the soft information 108 output from the demapper 102 may be referred to as ‘extrinsic LLRs’ and the soft information which is fed back to the demapper from the decoder may be referred to as ‘a priori LLRs’.
The embodiments described below are not limited to implementations which solve any or all of the disadvantages of known orthogonal frequency division multiplexing (OFDM) receivers.